// https://iai.sh.cn/problem/761
#include <bits/stdc++.h>
using namespace std;
int main() {
  int score;
  cin >> score;
  if (score >= 90) {
    cout << "A";
  } else if (score >= 80) {
    cout << "B";
  } else if (score >= 70) {
    cout << "C";
  } else if (score >= 60) {
    cout << "D";
  } else {
    cout << "F";
  }
  return 0;
}